Method and apparatus for determining the nature of subterranean reservoirs

ABSTRACT

A system for detecting or determining the nature of a subterranean reservoir. An electromagnetic field is applied using a dipole antenna transmitter and this is detected using a dipole antenna receiver. The measurements are taken with the antenna both in-line and parallel and the difference between the two sets of measurements is exploited. A characteristic difference indicates a high resistive layer, which corresponds to a hydrocarbon reservoir.

[0001] The present invention relates to a method and apparatus fordetermining the nature of submarine and subterranean reservoirs. Theinvention is particularly suitable for determining whether a reservoir,whose approximate geometry and location are known, contains hydrocarbonsor water, though it can also be applied to detecting reservoirs withparticular characteristics.

[0002] Currently, the most widely used techniques for geologicalsurveying, particularly in submarine situations, are seismic methods.These seismic techniques are capable of revealing the structure of thesubterranean strata with some accuracy. However, whereas a seismicsurvey can reveal the location and shape of a potential reservoir, itcannot reveal the nature of the reservoir.

[0003] The solution therefore is to drill a borehole into the reservoir.However, the costs involved in drilling an exploration well tend to bein the region of £25 m and since the success rate is generally about 1in 10, this tends to be a very costly exercise.

[0004] It is therefore an object of the invention to provide a systemfor determining, with greater certainty, the nature of a subterraneanreservoir without the need to sink a borehole.

[0005] It has been appreciated by the present applicants that while theseismic properties of oil-filled strata and water-filled strata do notdiffer significantly, their electromagnetic resistivities(permittivities) do differ. Thus, by using an electromagnetic surveyingmethod, these differences can be exploited and the success rate inpredicting the nature of a reservoir can be increased significantly.This represents potentially an enormous cost saving.

[0006] Consequently, a method and apparatus embodying these principlesfrom the basis of the present applicant's co-pending Internationalpatent application PCT/GB01/00419.

[0007] This contemplates a method of determining the nature of asubterranean reservoir whose approximate geometry and location areknown, which comprises: applying a time varying electromagnetic field tothe strata containing the reservoir; detecting the electromagnetic wavefield response; seeking in the wave field response, a componentrepresenting a refracted wave from the hydrocarbon layer; anddetermining the content of the reservoir, based on the presence orabsence of a wave component refracted by the hydrocarbon layer.

[0008] It also contemplates a method for searching for a hydrocarboncontaining subterranean reservoir which comprises: applying a timevarying electromagnetic field to subterranean strata; detecting theelectromagnetic wave field response; seeking, in the wave fieldresponse, a component representing a refracted wave; and determining thepresence and/or nature of any reservoir identified based on the presenceor absence of a wave component refracted by hydrocarbon layer.

[0009] It further contemplates an apparatus for determining the natureof a subterranean reservoir whose approximate geometry and location areknown, or for searching for a hydrocarbon containing subterraneanreservoir, the apparatus comprising: means for applying a time varyingelectromagnetic field to the strata containing the reservoir; means fordetecting the electromagnetic wave field response; and means forseeking, in the wave field response, a component representing arefracted wave, thereby enabling the presence and/or nature of areservoir to be determined.

[0010] A refracted wave behaves differently, depending on the nature ofthe stratum in which it is propagated. In particular, the propagationlosses in hydrocarbon stratum are much lower than in a water-bearingstratum while the speed of propagation is much higher. Thus, when anoil-bearing reservoir is present, and an EM field is applied, a strongand rapidly propagated refracted wave can be detected. This maytherefore indicate the presence of the reservoir or its nature if itspresence is already known.

[0011] Electromagnetic surveying techniques in themselves are known.However, they are not widely used in practice. In general, thereservoirs of interest are about 1 km or more below the seabed. In orderto carry out electromagnetic surveying as a stand alone technique inthese conditions, with any reasonable degree of resolution, shortwavelengths are necessary. Unfortunately, such short wavelengths sufferfrom very high attenuation. Long wavelengths do not provide adequateresolution. For these reasons, seismic techniques are preferred.

[0012] However, while longer wavelengths applied by electromagnetictechniques cannot provide sufficient information to provide an accurateindication of the boundaries of the various strata, if the geologicalstructure is already known, they can be used to determine the nature ofa particular identified formation, if the possibilities for the natureof that formation have significantly differing electromagneticcharacteristics. The resolution is not particularly important and solonger wavelengths which do not suffer from excessive attenuation can beemployed.

[0013] The resistivity of seawater is about 0.3 ohm-m and that of theoverburden beneath the seabed would typically be from 0.3 to 4 ohm-m,for example about 2 ohm-m. However, the resistivity of an oil reservoiris likely to be about 20-300 ohm-m. This large difference can beexploited using the techniques of the present invention.

[0014] Typically, the resistivity of a hydrocarbon-bearing formationwill be 20 to 300 times greater than water-bearing formation.

[0015] Due to the different electromagnetic properties of a gas/oilbearing formation and a water bearing formation, one can expect areflection and refraction of the transmitted field at the boundary of agas/oil bearing formation. However, the similarity between theproperties of the overburden and a reservoir containing water means thatno reflection or refraction is likely to occur.

[0016] Thus, an electric dipole transmitter antenna on or close to thesea floor induces electromagnetic (EM) fields and currents in the seawater and in the subsurface strata. In the sea water, the EM-fields arestrongly attenuated due to the high conductivity in the salineenvironment, whereas the subsurface strata with less conductivitypotentially can act as a guide for the EM-fields (less attenuation). Ifthe frequency is low enough (in the order of 1 Hz), the EM-waves areable to penetrate deep into the subsurface, and deeply buried geologicallayers having higher electrical resistivity than the overburden (as e.g.a hydrocarbon filled reservoir) will affect the EM-waves. Depending onthe angle of incidence and state of polarisation, an EM wave incidentupon a high resistive layer may excite a ducted (guided) wave mode inthe layer. The ducted mode is propagated laterally along the layer andleaks energy back to the overburden and receivers positioned on the seafloor. The term “refracted” wave in this specification is intended torefer to this wave mode.

[0017] Both theory and laboratory experiments show that the ducted modeis excited only for an incident wave with transverse magnetic (TM)polarisation (magnetic field perpendicular to the plane of incidence)and at angles of incidence close to the Brewster angle and the criticalangle (the angle of total reflection). For transverse electric (TE)polarisation (electric field perpendicular to the plane of incidence)the ducted mode will not be excited. Since the induced current isproportional to the electric field the current will be parallel to thelayer interfaces for TE polarisation but, for TM polarisation, there isan appreciable current across the layer interfaces.

[0018] A horizontal dipole source on the sea floor will generate both TEand TM waves, but by varying the orientation of the receiver antennae,it is possible to vary the sensitivity to the two modes of polarisation.It appears that an in-line orientation (source and receiver dipolesin-line) is more sensitive to the TM mode of polarisation, whereas aparallel orientation (source and receiver dipoles in parallel) is moresensitive to the TM mode of polarisation. The TM mode is influenced bythe presence of buried high resistive layers, whereas the TE mode isnot. By measuring with the two antenna configurations and exploiting thedifference between the two sets of measurements, it is possible toidentify deeply buried high resistivity zones, i.e. a hydrocarbonreservoir.

[0019] The present invention has arisen from this realisation.

[0020] According to one aspect of the present invention, there isprovided, a method of determining the nature of a subterranean reservoirwhich comprises: deploying an electric dipole transmitter antenna withits axis generally horizontal; deploying an electric dipole receiverantenna in-line with the transmitter; applying an electromagnetic (EM)field to the strata containing the reservoir using the transmitter;detecting the EM wave field response using the receiver and identifyingin the response a component representing a refracted wave from thereservoir according to a first mode; deploying an electric dipolereceiver antenna parallel to the transmitter; applying an EM field tothe strata using the transmitter; detecting the EM wave field responseusing the receiver and identifying in the response a componentrepresenting a refracted wave from the reservoir according to a secondmode; and comparing the first mode refractive wave response with thesecond mode refracted wave response in order to determine the nature ofthe reservoir.

[0021] According to another aspect of the present invention there isprovided, a method of searching for a hydrocarbon-containingsubterranean reservoir which comprises: deploying an electric dipoletransmitter antenna with its axis generally horizontal; deploying anelectric dipole receiver antenna in-line with the transmitter; applyingan EM field to subterranean strata using the transmitter; detecting theEM wave field response using the receiver; seeking in the response acomponent representing a refracted wave according to a first mode,caused by a high-resistivity zone; deploying an electric dipole receiverantenna parallel to the transmitter; applying an EM field to the stratausing the transmitter; detecting the EM wave field response using thereceiver; seeking in the response a component representing a refractedwave according to a second mode; and comparing the first mode refractivewave response with the second mode refractive wave response in order todetermine the presence and/or nature of any high-resistivity zone.

[0022] The first mode may be considered to be a TM mode, and the secondmode a TE mode.

[0023] Thus, according to the invention, measurements are taken with thetransmitter and receiver both in-line and parallel, and the two sets ofmeasurements are compared. A characteristic difference in valuesindicates a highly resistive layer located beneath highly conductivestrata. High resistivity indicates the presence of hydrocarbons and sothe difference in values is a direct hydrocarbon indicator.

[0024] This technique can be used in conjunction with conventionalseismic techniques to identify hydrocarbon reservoirs.

[0025] Preferably, the transmitter and/or receiver comprises an array ofdipole antennae.

[0026] The technique is applicable in exploring land-based subterraneanreservoirs but is especially applicable to submarine, in particularsub-sea, subterranean reservoirs. Preferably the field is applied usingone or more transmitters located on the earth's surface, and thedetection is carried out by one or more receivers located on the earth'ssurface. In a preferred application, the transmitter(s) and/or receiversare located on or close to the seabed or the bed of some other area ofwater.

[0027] In a preferred arrangement, the transmitter and receiver antennaeare located on a common cable towed behind a vessel. This will result ina fixed offset or a series of fixed offsets where several receivers areemployed. Preferably, the transmitter transmits both modes and maytherefore comprise two dipoles arranged mutually at right angles.Preferably each receiver comprises two dipoles mutually at right angles.Preferably one transmitter dipole and one receiver dipole are arrangedat right angles to the direction of the cable. Alternatively thetransmitter and/or receivers may each comprise a single dipole antennaarranged obliquely, eg. at 45° to the direction of the cable. With thisarrangement the transmitted field is resolved.

[0028] Using this technique, it is possible to achieve comparableresults from the two modes as the same signal and offset are used. Itwill not matter greatly if the transmitter drifts in frequency oramplitude. Furthermore, reservoirs can be detected in real time. Thus,if the results show a difference in the two modes, this will stronglyindicate the presence of an H/C bearing reservoir and so a more detailedstudy can be made at once.

[0029] Such a system would generally use a single transmission sourceand several receivers, typically more than ten. The different offsetswould be suitable for detecting reservoirs at different depths.

[0030] The receivers can be deployed on a single cable or on a series ofparallel cables. There may also be several transmitters.

[0031] In practice, the vessel would normally stop and the cable allowedto sink prior to transmission. There would be a transmission at severaldifferent frequencies before moving to another location. The techniqueis particularly suitable for edge detection, and it is a simple matterto select a suitable resolution. However, if the surveying is beingcarried out in an undetermined area, the resistivity in the top layersshould be mapped, for example with MT methods or by inversion after areflection study.

[0032] If the offset between the transmitter and receiver issignificantly greater than three times the depth of the reservoir fromthe seabed (i.e. the thickness of the overburden), it will beappreciated that the attenuation of the refracted wave will often beless than that of direct wave and the reflected wave. The reason forthis is the fact that the path of the refracted wave will be effectivelydistance from the transmitter down to the reservoir i.e. the thicknessof the overburden, plus the offset along the reservoir, plus thedistance from the reservoir up to the receivers i.e. once again thethickness of the overburden.

[0033] The polarization of the source transmission will determine howmuch energy is transmitted into the oil-bearing layer in the directionof the receiver. A dipole antenna is therefore the selected transmitter.In general, it is preferable to adopt a dipole with a large effectivelength. The transmitter dipole may therefore be 100 to 1000 meters inlength and may be towed in two orthogonal directions. The receiverdipole optimum length is determined by the thickness of the overburden.

[0034] The transmitted field may be pulsed, however, a coherentcontinuous wave with stepped frequencies is preferred. It may betransmitted for a significant period of time, during which thetransmitter should preferably be stationary (although it could be movingslowly), and the transmission stable. Thus, the field may be transmittedfor a period of time from 3 seconds to 60 minutes, preferably from 3 to30 minutes, for example about 20 minutes. The receivers may also bearranged to detect a direct wave and a wave refracted from thereservoir, and the analysis may include extracting phase and amplitudedata of the refracted wave from corresponding data from the direct wave.

[0035] Preferably, the wavelength of the transmission should be in therange

0.1s≦λ≦5s;

[0036] where λ is the wavelength of the transmission through theoverburden and s is the distance from the seabed to the reservoir. Morepreferably λ is from about 0.5s to 2s. The transmission frequency may befrom 0.01 Hz to 1 kHz, preferably from 1 to 20 Hz, for example 5 Hz.

[0037] Preferably, the distance between the transmitter and a receivershould be in the range

0.5λ≦L<10λ;

[0038] where λ is the wavelength of the transmission through theoverburden and L is the distance between the transmitter and the firstreceiver.

[0039] It will be appreciated that the present invention may be used todetermine the position, the extent, the nature and the volume of aparticular stratum, and may also be used to detect changes in theseparameters over a period of time.

[0040] The present invention also extends to a method of surveyingsubterranean measures which comprises; performing a seismic survey todetermine the geological structure of a region; and where that surveyreveals the presence of a subterranean reservoir, subsequentlyperforming a method as described above.

[0041] The invention may be carried into practice in various ways andwill now be illustrated in the following embodiments and reduced scaleinvestigations and simulations. In the accompanying drawings,

[0042]FIG. 1 is a vertical cross-section through a testing tank;

[0043]FIG. 2 is a plan view of the tank of FIG. 1;

[0044]FIG. 3 is a plan view of the antennae used in the tank of FIG. 1;

[0045]FIG. 4 is a side view of the antenna in FIG. 3;

[0046]FIGS. 5 and 6 are respectively a schematic plan view and side viewof the testing tank set up for measurement;

[0047]FIG. 7 is a graph showing calculated and measured values for thetransmitted electric field for a given frequency in the modelexperiment;

[0048]FIG. 8 is a graph showing calculated values for the electric fieldin a realistic earth model;

[0049]FIG. 9 is a schematic side view of a cable layout towed by avessel;

[0050]FIG. 10 is a plan view corresponding to FIG. 9; and

[0051]FIGS. 11 and 12 are views similar to FIG. 10 showing twoalternative arrangements.

[0052] The tank 11 shown in FIGS. 1 and 2 comprises a concrete enclosure9 m long, 6 m wide and 8 m in depth. The tank 11 is filled with seawater 12. A diaphragm 13 filled with fresh water 14 is located in thetank. The diaphragm 13 is 7.5 m long, 4.25 m wide and 0.25 m thick andcan be located at any desired height in a horizontal orientation withinthe tank 11.

[0053] The conductivity of the sea water 12 was measured to be 5.3 S/mat 14° C. and the conductivity of the fresh water was measured to be0.013 S/m. The ratio of the two conductivities is therefore very closeto 400.

[0054] The critical frequency f_(c) of a conducting medium, i.e. thefrequency at which the displacement current is equal to the conductioncurrent, is given by$f_{c} = {\frac{\sigma}{2\quad \pi \quad ɛ_{r}ɛ_{o}} \approx {{18 \cdot \frac{\sigma}{ɛ_{r}}}\quad {GHz}}}$

[0055] where ε_(r) is the relative dielectric constant of the medium,and σ the conductivity in S/m. For water, ε_(r)=80 at the frequenciesand temperatures of interest. For the two conductivity values σ=5.2 S/mand σ=0.013 S/m, f_(c)=1.2 GHz and 3 Mhz, respectively. Since, in theexperiments, the highest frequency is 0.83 MHz, it is a fairapproximation to neglect the displacement current, even for the freshwater.

[0056] For a non magnetic, conductive medium, the propagation constant γis given by $\begin{matrix}{\gamma = {{j\quad \omega \sqrt{ɛ\quad \mu}} = {j\quad \omega \sqrt{\left( {{ɛ_{r}ɛ_{o}} + \frac{\sigma}{j\quad \omega}} \right)\mu_{o}}}}} \\{{\approx \sqrt{j\quad \omega \quad \mu_{o}\sigma}} = {2\pi \sqrt{\frac{{j \cdot f}\quad \sigma}{5}}}}\end{matrix}$

[0057] The wavelength λ, defined as the distance in which the phasechanges 2π, is given by$\lambda = {\frac{2\quad \pi}{{Re}\left\{ \gamma \right\}} \approx \sqrt{\frac{10}{f\quad \sigma}}}$

[0058] λ m, f in MHz and σ in S/m. The skin depth, the distance in whichthe amplitude diminishes by 1/e, is related to the wavelength by$\delta = {\frac{\lambda}{2\quad \pi}\sqrt{\frac{\pi}{2\quad f\quad \mu_{o}\sigma}}}$

[0059] For the extremes of the frequency range, for the sea water withσ=5.2 S/m, Frequency 30 kHz 830 kHz Skin depth 1.27 m 0.24 m Wavelength8.01 m 1.52 m

[0060] and for the fresh water with σ=0.013 S/m, Frequency 30 kHz 830kHz Skin depth  25.4 m  4.8 m Wavelength 160.2 m 30.4 m

[0061] Referring now to FIGS. 3 and 4, two identical electrical dipoleantennae, as shown were used for the transmitter and receiver.

[0062] Each antenna 15 comprises two square brass plates 16, 15 cmsquare, mounted on an epoxy substrate 17. Each plate 16 is connected toa co-axial cable 18, which passes through an epoxy tube 19 mounted atright angles to the plate 16, to a balun which transforms the impedanceof the antenna 15 from about 2 Ω in sea water to about 50 Ω.

[0063] The measurement set-up is shown in FIGS. 5 and 6. An automaticnetwork analyser (ANA) measures the transmission between the antennae 15as a function of distance (offset) and frequency. The arrangement shownin FIG. 5 shows the antennae 15 in the parallel orientation. The in-lineorientation is achieved by rotating both antennae through 90° in thehorizontal plane.

[0064] The results of the measurements are shown in FIG. 7 together withthe corresponding theoretical results. The measurements agree well withthe theoretical results and the figure contains two sets of curves, onewith parallel antennae and one with the antennae in line. Thetheoretical results are computed for infinitesimal dipole antennae. Theorientations of the antennae and the frequency are shown on the Figures.

[0065] The parameters of the experiment are scaled relative to possiblepractical situations. To give an idea of orders of magnitude; if thefrequency is scaled down by a factor of 40,000 and the conductivity by afactor of 10, the dimensions will be scaled up by a factor of 632, andthe experimental setup would correspond to a low conductivity layer ofthickness 150 m and conductivity 0.0013 S/m below an overburden ofthickness 300 m and conductivity 0.52 S/m. The corresponding frequencyrange would be from 0.75 Hz to 20 Hz, and the length of the antennanearly 300 m.

[0066] The method with the TE and TM-mode have been tested by computersimulations on a simple horizontally layered earth model with electricalparameter values for typical deep water subsurface sediments. The modelhas an infinite insulating air layer, a 1150 meter water layer of 0.3125Ωm. 950 meter overburden of 1 Ωm, a 150 meter reservoir zone of 50 Ωmand an infinite underburden of 1 Ωm. FIG. 8 illustrates the amplituderesponse |E| (electric field) as a function of receiver offset, causedby a 1 Hz signal. Responses from both the TM-mode (solid with x's) andTE-mode (dashed with +'s) are shown. The amplitudes for the TM-mode areapproximately 10 times larger at an offset of 5 km. As a reference, theresponse from a homogenous half-space of 1 Ωm is shown for bothconfigurations (corresponding to a response from a water fillerreservoir or outside the reservoir area). The TE-mode has the largestdeviation from its half-space, ie. this mode is more sensitive to ahydrocarbon layer.

[0067]FIGS. 9 and 10 show a vessel 31 towing a cable (or streamer) 32just above the seabed 33. The cable 32 carries a transmitter dipoleantenna 34 and several receiver dipoles 35, only four of which areshown. The depth of water might be of the order of 1000 m, the offsetbetween the transmitter 34 and the nearest receiver 35 might be about2000 m and the receivers might be about 100 m apart. The transmitter 34is controlled from the vessel 31 via the cable 32 and the responsesdetected by the receivers 35 are relayed back to the vessel 31 in realtime, again via the cable 32.

[0068]FIG. 10 shows an arrangement in which the vessel 31 tows threecables 41, 42, 43, each carrying a series of receivers 45, 46, 47. Thespacing of the three cables 41, 42, 43 is achieved by means of a spar44.

[0069] In the arrangement shown in FIG. 11, the transmitter 48 is in theform of two dipole antennae one parallel to the cable 42 and one atright angles.

[0070] The arrangement shown in FIG. 12 is similar to FIG. 11, but inthis case, the transmitter 51 is a single dipole antenna arranged at 45°to the cable 42.

1. A method of determining the nature of a subterranean reservoir which comprises: deploying an electric dipole transmitter antenna with its axis generally horizontal; deploying an electric dipole receiver antenna in-line with the transmitter; applying an electromagnetic (EM) field to the strata containing the reservoir using the transmitter; detecting the EM wave field response using the receiver and identifying in the response a component representing a refracted wave from the reservoir according to a first mode; deploying an electric dipole receiver antenna parallel to the transmitter; applying an EM field to the strata using the transmitter; detecting the EM wave field response using the receiver and identifying in the response a component representing a refracted wave from the reservoir according to a second mode; and comparing the first mode refractive wave response with the second mode refracted wave response in order to determine the nature of the reservoir.
 2. A method of searching for a hydrocarbon-containing subterranean reservoir which comprises: deploying an electric dipole transmitter antenna with its axis generally horizontal; deploying an electric dipole receiver antenna in-line with the transmitter; applying an EM field to subterranean strata using the transmitter; detecting the EM wave field response using the receiver; seeking in the response a component representing a refracted wave according to a first mode, caused by a high-resistivity zone; deploying an electric dipole receiver antenna parallel to the transmitter; applying an EM field to the strata using the transmitter; detecting the EM wave field response using the receiver; seeking in the response a component representing a refracted wave according to a second mode; and comparing the first mode refractive wave response with the second mode refractive wave response in order to determine the presence and/or nature of any high-resistivity zone.
 3. A method as claimed in claim 1 or claim 2, characterised in that the first mode is a TM mode of polarisation and/or the second mode is a TE mode of polarisation.
 4. A method as claimed in any preceding claim, characterised in that the transmitter and/or receiver comprises an array of dipole antennae.
 5. A method as claimed in any preceding claim, characterised in that the transmitter and/or receiver is located on or close to the seabed or the bed of some other area of water.
 6. A method as claimed in any preceding claim, characterised in that the transmitter and receivers are located on a common cable arranged to be towed behind a vessel.
 7. A method as claimed in any preceding claim, characterised in that the transmitter comprises two dipole antennae arranged mutually at right angles.
 8. A method as claimed in any preceding claim, characterised in that each receiver comprises two dipole antennae arranged mutually at right angles.
 9. A method as claimed in claim 6 characterised in that the transmitter and/or receiver each comprise a single dipole antenna arranged obliquely to the direction of the cable.
 10. A method as claimed in any preceding claim, characterised in that the frequency of the EM field is continuously varied over the transmission period.
 11. A method as claimed in any preceding claim, characterised in that the field is transmitted for a period of time for 3 seconds to 60 minutes.
 12. A method as claimed in claim 11, characterised in that the transmission time is from 3 to 30 minutes.
 13. A method as claimed in any preceding claim, characterised in that the wavelength of the transmission is given by the formula 0.1s≦λ≦10s; wherein λ is the wavelength of the transmission through the overburden and s is the distance from the seabed to the reservoir.
 14. A method as claimed in any preceding claim, characterised in that distance between the transmitter and a receiver is given by the formula 0.5≦λ≦10s; where λ is the wavelength of the transmission through the overburden and 1 is the distance between the transmitter and the receiver.
 15. A method as claimed in any of claims 10 to 14, characterised in that the transmission frequency is from 0.01 Hz to 1 kHz.
 16. A method as claimed in claim 15, characterised in that the transmission frequency is from 1 to 20 Hz.
 17. A method as claimed in any preceding claim, characterised in that it includes suppressing the direct wave and/or any other known wave contribution that may disturb the measurements, thereby reducing the required dynamic range of the receiver and increasing the resolution of the refracted wave.
 18. A method of surveying subterranean measures which comprises: performing a seismic survey to determine the geological structure of a region and where that survey reveals the presence of a subterranean reservoir, subsequently performing a method as claimed in any preceding claim. 